Microscopic Polyangiitis

Question 1

Microscopic polyangiitis

Answer 1

Microscopic polyangiitis (MPA) is one of the anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitides (AAV) that can present with rapidly progressive renal impairment and other systemic manifestations.

In the UK, the incidence is estimated at 2 cases per 100,000 people. It is more common among older Caucasians with an equal sex distribution.

Question 2

… is characterised by a necrotising vasculitis without granulomas.

Answer 2

MPA is characterised by a necrotising vasculitis without granulomas.

Question 3

GPA VS MPA

Answer 3

MPA is characterise by the presence of inflammation in small vessels with pauci-immune (minimal evidence of hypersensitivity under the microscope), necrotising vasculitis without evidence of granulomas. The presence of granulomas would be supportive of GPA.

This vasculitis may affect multiple organ systems including ears, nose and throat, lungs, kidneys, skin, eyes and nervous system. Both MPA and GPA are closely related conditions based on clinical presentations and organs involved.

Question 4

Over the years, there have been many attempts to define and classify AAV and to differentiate between MPA, GPA and EGPA.

Based on CHCC, MPA is defined as:

Answer 4

“Necrotizing vasculitis, with few or no immune deposits, predominantly affecting small vessels (ie, capillaries, venules, or arterioles). Necrotizing arteritis involving small and medium arteries may be present. Necrotizing glomerulonephritis is very common. Pulmonary capillaritis often occurs. Granulomatous inflammation is absent.”

Question 5

Therefore, the key differentiation between MPA and GPA is the absence of granulomatous inflammation on biopsy.

… = no granulomatous inflammation

Answer 5

Therefore, the key differentiation between MPA and GPA is the absence of granulomatous inflammation on biopsy.

MPA = no inflammation

Question 6

Diagnosis if based on presence of typical clinical features, ANCA positivity and characteristic biopsy findings.
The key investigations to confirm the diagnosis of suspected AAV are biopsy and ANCA serology

Answer 6

Biopsy: should be targeted to affected organ (e.g. nose, kidneys). Presence or absence of granulomatous inflammation helps differentiate MPA from GPA.
ANCA: urgent serology should be taken to see whether specificity towards PR3 or MPO is present.

Question 7

Prognosis

MPA

Answer 7

The five year survival rate of MPA is 75%.
It is estimated that >90% will improve with treatment but only 75% will achieve disease remission. There is a risk of relapse in 30% of patient by two years.